On my campus 3 out of the 4 fraternities have lil sis programs, only 1 allows their lil sis's to wear their letters. (I just don't believe in that...unless you're pinned...just my opinion)
If you're a single girl when you become a lil sis you're more of a "rush tool", if you're hot, rushees will naturally go towards the fraternity w/the hottest girls that hang around. (You know it's true!
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Of the sororities, 3 of the 5 have lil bro programs, mine included. I have a lil bro named Matt who is a TKE from Shawnee Sate, he's my boy and I love to spoil him! I also have a big bro in a fraternity, it's not that serious though, at least for them. When we have lil bro's go through they wear a "Lambda loincloth", hehehe, gotta love it
Some orgs here take it pretty seriously and have an actual pledging process they have to go through. Us, well, it last about a couple days and is more fun than anything.